Overview of Hard Disk Technology

Hard disks contain several disks called platters, these platters are mounted on a spindle. The platters rotate at speeds between 2000 and 10000 rpm.
The data stored on the platters is read by several read/write heads, which are all mounted on a single actuator. Each side of each platter has its own head which floats above the platters surface.

Each platter is divided up into sectors and tracks. Sectors are shaped like slices of cake, whereas tracks are concentric circles running around the platter.
Tracks start out the outside edge. The first track contains the File Allocation Table (FAT).
Because hard disks contain more than one platter, and because the read/write heads on all platters read the same tracks on each disk, the collection of tracks the read/write heads access at any one point is called a cylinder.

Hard Drive Geometry

A hard drives geometry is essentially the spec of the hard drive which dictates how much data the drive can store and how quickly it can access it. Hard disks are rated by several variables:

• Capacity
• Seek time
• Latency
• Access time
• Spin speed

A hard drives capacity can be calculated using the following formula:

Capacity = (No. Cyl x No. Heads) x (No. Sectors per track) x (Capacity per track)

Seek Time - This is the time it takes for the actuator to move from rest to the correct point on the disk.

Latency Factor - This is the time it takes for the correct track to pass underneath the read/write heads, the faster the spin speed the lower the latency factor.

Access Time - This is a found by adding the average seek time and average latency factors together.